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Lesson 11

Lesson 11. Nuclear Reactions --Part 2. Compound Nuclear Reactions. The compound nucleus is a long-lived reaction intermediate that is formed by a complex set of interactions between the projectile and the target.

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Lesson 11

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  1. Lesson 11 Nuclear Reactions --Part 2

  2. Compound Nuclear Reactions • The compound nucleus is a long-lived reaction intermediate that is formed by a complex set of interactions between the projectile and the target. • The projectile and target nuclei fuse and the energy of the projectile is shared among all the nucleons of the composite system. • The lifetime of the CN is ~ 10-18-10-16 s. This lifetime is directly measurable by crystal-blocking techniques.

  3. Independence Hypothesis or “Amnesia Assumption” • The mode of decay of the CN is independent of its mode of formation. • Caveat: Conservation laws apply. • Experimental evidence: the Ghoshal expt. • Angular distributions symmetric about 90 in CN frame.

  4. 62Cu>63Zn>62Ni ang. mom effects

  5. Cross Sections--General

  6. Cross Sections--Energy Dependence

  7. /D << 1 For reaction a + A Cb + B Applying this to (n,) reactions

  8. /D >>1 For reaction a + A Cb + B ab = C PC(b)

  9. /D >>1 • Assumption is that of statistical equilibrium • For reaction a + A Cb + B ab = C PC(b) • So the problem is to calculate the probability that C will decay to B + b.

  10. Level densities

  11. If emitted particles are neutrons

  12. If emitted particles are charged particles

  13. Excitation functions

  14. Photonuclear Reactions

  15. Photonuclear Reactions • GDR-- a giant oscillation of the nuclear protons vs the nuclear neutrons • dipole sum rule

  16. Heavy Ion Reactions • Classical motion • Dominated by high angular momentum • Aproj > 4

  17. Mechanisms

  18. Mechanisms

  19. Elastic scattering

  20. Fusion

  21. Deep Inelastic Scattering

  22. High Energy Reactions • Low energy (< 10 MeV/A) • Intermediate Energy ( 20-250 MeV/A) • High Energy (>250 MeV/A)

  23. Spallation

  24. New high energy mechanisms • Spallation • IMF formation • Cascade processes • Participant-Spectator Picture

  25. Cascades nucleon-nucleon collisions

  26. Participant-Spectator Physics

  27. Multifragmentation • “Multifragmentation” refers to central collisions where several IMFs are emitted. • Caloric curve

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